The present invention relates to an image pickup device using solid-state imaging elements, such as charge-coupled devices (CCDs), and more particularly to a driving method and driving device which perform pixel addition by special driving.
In recent years, various digital cameras using solid-state imaging elements, such as CCDs, have been developed. In a digital camera, the photoelectric conversion portion converts an image of the subject photoelectrically, thereby producing a pixel signal (charge). The pixel signal is transferred over a transfer path including CCDs and then outputted. To read the pixel signal from the image pickup device, a driving method of reading the charge in each pixel line by line is generally used. In addition, other special types of driving have been used.
One known typical special driving is n-times-speed vertical-addition driving (hereinafter, just referred to as n-addition driving), a driving method for high-speed, high-sensitivity reading. In “n-addition driving,” the number of pixels (the number of transfer clocks) transferred from the vertical transfer path to the horizontal transfer path in each horizontal (H) blanking interval is set to an integer “n” equal to or larger than 2, not to 1 as is usual, which enables charges of n pixels (or n lines) to be transferred sequentially to the horizontal transfer path and then the charges of n pixels (or n lines) added in the horizontal transfer path to be read as a single pixel of one line.
This reduces the number of lines (horizontal lines) on one screen to 1/n, with the result that the reading time of one screen decreases to 1/n, enabling high-speed reading. Since the charges of n pixels are read as the charge of one pixel, the amount of charge of one pixel increases n times as much and accordingly the sensitivity is increased.
When a high-luminance subject is imaged, however, n-addition driving may cause a white-striped pseudo signal (a phenomenon similar to blooming or a smear, in this specification, referred to as horizontal streak noise) in the horizontal direction, which will possibly deteriorate the picture quality.
The phenomenon will be explained below.
If the saturated level (or the maximum amount of charge transmittable) of the horizontal transfer path in which the charges of a plurality of pixels are added were infinite, there would be no problem. Actually, however, the saturated level is finite. Let the saturated level be expressed as SatH. Normally, SatH is designed to correspond to the saturated level of the photoelectric conversion portion (for one pixel) in the normal driving state, non-addition driving. The saturated level of the photoelectric conversion unit, in other words, is the overflow level OFL of the charge accumulating portion. Even when charges exceeding the level are produced, they are discharged to the overflow drain and not accumulated. The overflow level OFL can be varied according to the set value of the substrate bias voltage VSUB explained later. Since setting the overflow level OFL too high makes blooming liable to occur, setting is done so that the overflow level OFL may be as high as possible in the range of the tolerance limit in the blooming characteristic.
Specifically, the saturated level SatH of the horizontal transfer path generally takes such a value as gives some design margins or adjustment margins to the standard setting of the overflow level OFL of the charge accumulating portion, and does not exceed that value:SatH=k×OFL  (1)where k=about 1.1 to 1.5 (theoretical lower limit is 1.0).
When n-addition driving is performed, the pixel signal is made n times as large by addition, thereby inputting the charges exceeding the saturated level SatH to the horizontal transfer path. Specifically, even when the amount of charge per pixel is equal to or below the overflow level OFL of the charge accumulating portion, if imaging is done with such a high luminance as exceeds SatH/n, n-addition will permit charges exceeding the saturated level SatH to be transferred to the horizontal transfer path. If satisfactory excess charge measures (for example, setting an overflow drain) have been taken for the horizontal transfer path, the input of such excess charges to the horizontal transfer path will just cause the signal charge to be clipped at the saturated level SatH, causing no problem. In some actual CCDs, unsatisfactory excess charge measures have been taken for the horizontal transfer path, permitting the excess charges to overflow into the regions adjacent to the horizontal transfer path, which causes horizontal streak noise along the horizontal lines.